1. Field of the Invention
The invention relates generally to optical frequency shifters, and more particularly to a method and apparatus for frequency shifting an optical beam by the use of a phase plate or a spiral wheel.
2. Description of the Prior Art
The prior art uses a variety of methods and apparatus for optical frequency shifting. For example, the prior art utilizes the surface of a rotating wheel to scatter light in order to effect a frequency shift. The scattering of this method and apparatus does provide a Doppler shift, but it also yields a scattered, as distinct from a reflected, beam of light. This beam cannot be used as a local oscillator because of the loss of spatial coherence. Also, the scattered beam has a spread of Doppler shifts because different parts of the illuminated spot on the wheel have different line-of-sight velocities, and if a range of scattered angles are collected, they all have different Doppler shifts even if they all come from essentially the same location on the surface of the wheel because of the different angles between the scattered direction and the velocity of the scattering center.
Other methods include the use of a vibrating mirror or electro-optical phase modulator, which can be used to provide a frequency shifted beam for only a short period of time because such systems must be reset and repeated after adding only about one to 10 wavelengths of optical path compared to about one thousand wavelengths that can be added in the present invention before it must be reset.
Prior art systems employing electro-optic amplitude modulation impress frequency sidebands on the beam. These sidebands can be shifted up to about 100 MHz, but the modulator does not spatially separate them from each other or from the unshifted beam.
Acousto-optic modulation utilizes radio frequencies to set up a traveling acoustic wave in a germanium crystal. The light is then reflected off of the changes in index of refraction that are caused by the density waves. This can be up to 80% efficient, but the direction of the reflected beam depends on the frequency shift imposed. Frequency shifts of 40 MHz to 110 MHz can be obtained by this method.
Furthermore, Raman scattering can provide large, fixed frequency shifts on the order of 50 GHz. Other techniques, such as parametric amplification and other non-linear processes, have been tried but are usually of such low efficiency and are so difficult to implement that they are not generally used.
The present invention solves substantially all of the problems of the prior art while avoiding most of its short comings.